Feel++

Post-Processing and Visualization

Introduction

Once the PDE is solved we usually would like to visualize the solution and possibly other data or fields associated with the problem. Feel++ provides a very powerful framework for post-processing allowing to:

  • visualize time-based data,

  • visualize high order functions,

  • visualize traces,

  • use standard visualization tools such as Paraview, Ensight and Gmsh.

To achieve this, Feel++ defines a so-called Exporter object.

General principles

The library Feel itself does not have any visualization capabilities. However, it provides tools to export both scalar and vector fields into two formats: EnSight and Gmsh. The EnSight format can be read by the visualization software EnSight or, for instance, by the open source package Paraview.

The choice of format depends on several factors, some of them being the robustness/capabilities of the visualization packages associated and the type of data to be plotted.

For first (at most second) degree piecewise polynomials defined in straight edge/faces meshes, Paraview (or any other software that reads the EnSight format) is a very good choice. A trick to use Paraview (or any other visualization software that plots first order geometrical and finite elements) to visualize high degree polynomials in curved meshes, is to use an interpolation operator built on high order nodes associated with the polynomial space, see~\cite gpena_cprudhomme_acomen. However, for high degree polynomials or meshes with curved elements, Gmsh is prefered due to its adaptive visualization algorithm for this type of finite/geometrical element.

To illustrate both approaches in visualizing high degree polynomials in curved meshes, we plot, in the Figures below, the nodal projection of the function f(x,y)=\cos(5x) \sin(5y) in the unit circle onto several function spaces. Notice the improved look of the projection using high degree polynomials instead of linear projections in a finer mesh. We remark also that the difference between the two approaches fades away as we increase the degree of the polynomials and the order of the geometrical elements. However, the additional cost of building the piecewise first order finite element space associated with the finer mesh, the calculation of the projection onto this space and the smoothness of the graphics makes Gmsh’s approach more appealing to visualize this type of finite elements. We highlight that these algorithms are available for meshes composed only of simplices or quadrilaterals (see \ref Notations), in 1D, 2D and 3D.

Table 1. Comparison between various geometrical interpolation order

Order 1

Order 2

P2isoP1

Order 3

P3isoP1

Order 4

P4isoP1

Order 5

P5isoP1

circle p1p1

circle p2p2

circle p2p2 p1interpolator

circle p3p3

circle p3p3 p1interpolator

circle p4p4

circle p4p4 p1interpolator

circle p5p5

circle p5p5 p1interpolator

To further illustrate the capabilities of the Feel’s exporter to Gmsh, we plot in Figures 1D and 3D functions defined on the unit sphere.

Table 2. Visualisation of a function using geometrical 1 to 4 in 3D

Order 1

Order 2

Order 3

Order 4

sphere p1

sphere p2

sphere p3

sphere p4

Visualization

We would like to visualize the function u=\sin(\pi x) over \Omega= \{ (x,y) \in \mathbb{R}^2 | x^2 + y^2 < 1 \} \Omega is approximated by \Omega_h.

To define \Omega and u the code reads mesh

Unresolved directive in exporter.adoc - include::../../../../codes/05-myexporter.cpp[tag=exporter]

We start with an Exporter object that allows to visualize the P_1 interpolant of u over \Omega.

ExporterReference Reference

Options

exporter.format

  • Type: multiple choice string

  • Values: gmsh , ensight , ensightgold

  • Default value: ensightgold

  • Action: exporter.format defines the format to save Feel++ data into.

exporter.geometry

  • Type: multiple choice string

  • Values: change_coords_only , change , static

  • Default value: change_coords_only

  • Action: exporter.geometry tells to the exporter if the mesh changes over time steps : no changes (static) coordinates only (change_coords_only) or remeshed (changes)

exporter.fileset

  • Type: bool

  • Values: 0, 1

  • Default value: 0

  • Action: exporter.fileset=0 save one file per timestep per subdomain, whereas exporter.fileset=1 use one file per subdomain to store all time steps.

This option, exporter.fileset=1, reduces tremendously the number of files generated for transient simulations.

exporter.prefix

  • Type: string

  • Default Value: <empty string>

  • Action: exporter.prefix defines the prefix to be user by the exporter. It is especially useful when using multiple exporters and avoid name collision.

exporter.directory

  • Type: string

  • Default Value: results

  • Action: exporter.directory tells where to export the results

Ensight Gold specific options

exporter.ensightgold.use -sos

  • Type: bool

  • Action: if exporter.ensightgold.use -sos=0 multiple case files are handle in first case file else the sos file is used to handle multiple case files

exporter.ensightgold.save -face

  • Type: bool

  • Action: if exporter.ensightgold.save -face=1, the exporter saves mesh and fields on marked faces